The present invention relates to a phase modulator circuit, more particularly to an edge modulator circuit. Such phase modulators have particular, but not exclusive application, in frequency synthesiser circuits for use in communications equipment and precision test equipment, and in analog to digital converters.
Phase modulators are generally well known and three examples are disclosed in U.S. Pat. No. 2,441,418. In all three examples, the circuit comprises a first triode valve having a differentiating transformer in its anode circuit, a saw-tooth wave generator connected to its grid and a modulator tube connected to its cathode. The modulator tube comprises a second triode valve the grid-cathode bias of which, and thereby the conductivity of the second valve, is varied in accordance with an externally applied modulating signal. The anode of the second valve is connected to the cathode of the first valve and thereby the second valve controls the conductivity of the first valve in accordance with the modulating signal. A capacitor is connected in parallel with the anode-cathode current path of the second valve. The capacitor stores the charge resulting from the first valve being rendered conductive. During the time interval between successive saw-tooth waves, the charge on the capacitor leaks away via the second valve and/or a resistor connected in parallel with the capacitor.
In operation, the modulating signal applied to the second valve varies its grid-cathode bias and thereby the bias on the first tube, hence varying the critical value or time at which the first tube starts to conduct over the range of the applied saw-tooth wave. When the starting point of current flow in the first valve is varied by the modulating signal, the occurrence time of a resulting output pulse in the transformer is also varied with respect to the leading and trailing edges of the saw-tooth wave.
A problem with this known circuit is that if the capacitor has a high value, it takes a number of cycles of the saw-tooth wave to restore equilibrium if a change occurs in the circuit, this delay in restoring equilibrium renders the circuit unsuitable for high speed, precision operation. A further disadvantage of this known circuit is that the saw-tooth wave generator is free running so that the sampling of the modulating signal is fixed and cannot be controlled externally. Yet another disadvantage of this known phase modulator is that noise is produced in the saw-tooth wave generator, which noise may affect the detection of the level of the modulating signal, and noise may also arise on the modulating signal input which will cause a variable bias to be applied to the first valve thereby affecting its stability and response to changes.
In other known edge modulator circuits, a comparator is used to compare the value of a ramp with a modulating signal. The signal acts as a variable threshold and determines the time at which the comparator output changes state. These known edge modulators are affected adversely both by noise present on the modulating signal and by noise being produced in the ramp generator. Furthermore these known circuits are not generally suited to construction as integrated circuits.